1. Field of the Invention
The present invention relates to an electrophotographic copying apparatus, particularly, it relates to an electrophotographic copying apparatus designed to prevent poor image forming due to an abnormal scanning speed of an original.
2. Description of the Prior Art
Conventionally, in an electrophotographic copying apparatus, in order to scan and expose an original to project an image of the original on a photosensitive surface moving at a constant speed, an optical scanning system including a light source and the original are moved relatively. As a system for scanning and exposing the original, an original moving system in which the original itself or an original table carrying the original are moved and an optical system moving system in which the optical scanning system is moved relative to the original stationed are known. In either case, the scanning speed of the original must be constant. That is, the scanning speed must always be kept constant to obtain a good image since the fluctuations of scanning speed can cause the image formed on the photosensitive surface to deform.
Therefore, in Japanese Patent Application Laid Open No. 61-63831, a technique to stop the optical scanning system by detecting its moving speed as abnormal when it is lower than a predetermined value is disclosed.
It is definite that the technique disclosed Japanese Publication can prevent a deformed image from being outputted when the scanning speed fluctuates. However, among the causes of fluctuations of scanning speed, there are many causes which can be dissolved instantly. For example, such fluctuations of scanning speed as in the case where torque of a driving system fails due to a transient power voltage drop or the case, in the original moving system, where the user has applied excessive load on the original itself or on the original table are transient ones which can be resolved immediately. The aforesaid technique which merely stops scanning when the scanning speed has changed due to such a transient cause makes it inconvenient to operate the copying apparatus.
In order to maintain the scanning speed always constant, though it is also possible to detect the scanning speed and change the voltage value to be applied to the driving system responsive to the detected speed, a range which can be corrected by changing the applied voltage value is limited and exceeding the limit will also result in a poor image forming.